Android Advanced Topics

GDB

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Overview

Xamarin.Android 4.10 introduced partial support for using gdb by using the the _Gdb MSBuild target.

gdb support requires that the Android NDK be installed.

There are three ways to use gdb:

  1. Debug builds with Fast Deployment enabled .
  2. Debug builds with Fast Deployment disabled .
  3. Release builds .

When things go wrong, please see the Troubleshooting section.

Debug Builds with Fast Deployment

When building and deploying a Debug build with Fast Deployment enabled, gdb can be attached by using the _Gdb MSBuild target.

First, install the the app. This can be done via the IDE, or via the command line:

$ /Library/Frameworks/Mono.framework/Commands/xbuild /t:Install *.csproj

Secondly, run the _Gdb target. At the end of execution, a gdb command line will be printed:

$ /Library/Frameworks/Mono.framework/Commands/xbuild /t:_Gdb *.csproj
...
    Target _Gdb:
        "/opt/android/ndk/toolchains/arm-linux-androideabi-4.4.3/prebuilt/darwin-x86/bin/arm-linux-androideabi-gdb" -x "/Users/jon/Development/Projects/Scratch.HelloXamarin20//gdb-symbols/gdb.env"
...

The _Gdb target will launch an arbitrary launcher Activity declared within your AndroidManifest.xml file. To explicitly specify which Activity to run, use the RunActivity MSBuild property. Starting Services and other Android constructs is not supported at this time.

The _Gdb target will create a gdb-symbols directory and copy the contents of your target's /system/lib and $APPDIR/lib directories there.

Note: The contents of the gdb-symbols directory are tied to the Android target you deployed to, and will not be automatically replaced should you change the target. (Consider this a bug.) If you change Android target devices, you will need to manually delete this directory.

Finally, copy the generated gdb command and execute it in your shell:

$ "/opt/android/ndk/toolchains/arm-linux-androideabi-4.4.3/prebuilt/darwin-x86/bin/arm-linux-androideabi-gdb" -x "/Users/jon/Development/Projects/Scratch.HelloXamarin20//gdb-symbols/gdb.env"
GNU gdb (GDB) 7.3.1-gg2
...
(gdb) bt
#0  0x40082e84 in nanosleep () from /Users/jon/Development/Projects/Scratch.HelloXamarin20/gdb-symbols/libc.so
#1  0x4008ffe6 in sleep () from /Users/jon/Development/Projects/Scratch.HelloXamarin20/gdb-symbols/libc.so
#2  0x74e46240 in ?? ()
#3  0x74e46240 in ?? ()
(gdb) c

Debug Builds without Fast Deployment

Debug builds with Fast Deployment operate by copying the Android NDK's gdbserver program into the Fast Deployment .__override__ directory. When Fast Deployment is disbaled, this directory may not exist.

There are two workarounds:

  • Set the debug.mono.log system property so that the .__override__ directory is created.
  • Include gdbserver within your .apk.

Setting the debug.mono.log System Property

To set the debug.mono.log system property, use the adb command:

$ adb shell setprop debug.mono.log gc

Once the system property has been set, execute the _Gdb target and the printed gdb command, as with the Debug Builds with Fast Deployment configuration:

$ /Library/Frameworks/Mono.framework/Commands/xbuild /t:_Gdb *.csproj
  ...
    Target _Gdb:
        "/opt/android/ndk/toolchains/arm-linux-androideabi-4.4.3/prebuilt/darwin-x86/bin/arm-linux-androideabi-gdb" -x "/Users/jon/Development/Projects/Scratch.HelloXamarin20//gdb-symbols/gdb.env"
  ...
$ "/opt/android/ndk/toolchains/arm-linux-androideabi-4.4.3/prebuilt/darwin-x86/bin/arm-linux-androideabi-gdb" -x "/Users/jon/Development/Projects/Scratch.HelloXamarin20//gdb-symbols/gdb.env"
GNU gdb (GDB) 7.3.1-gg2
...
(gdb) c

Including gdbserver in your app

To include gdbserver within your app:

  1. Find gdbserver within your Android NDK (it should be in $ANDROID_NDK_PATH/prebuilt/android-arm/gdbserver/gdbserver), and copy it into your Project directory.
  2. Rename gdbserver to libs\armeabi-v7a\libgdbserver.so.
  3. Add libs\armeabi-v7a\libgdbserver.so to your Project with a Build action of AndroidNativeLibrary.
  4. Rebuild and reinstall your application.

Once the app has been reinstalled, execute the _Gdb target and the printed gdb command, as with the Debug Builds with Fast Deployment configuration:

$ /Library/Frameworks/Mono.framework/Commands/xbuild /t:_Gdb *.csproj
  ...
    Target _Gdb:
        "/opt/android/ndk/toolchains/arm-linux-androideabi-4.4.3/prebuilt/darwin-x86/bin/arm-linux-androideabi-gdb" -x "/Users/jon/Development/Projects/Scratch.HelloXamarin20//gdb-symbols/gdb.env"
  ...
$ "/opt/android/ndk/toolchains/arm-linux-androideabi-4.4.3/prebuilt/darwin-x86/bin/arm-linux-androideabi-gdb" -x "/Users/jon/Development/Projects/Scratch.HelloXamarin20//gdb-symbols/gdb.env"
GNU gdb (GDB) 7.3.1-gg2
...
(gdb) c

Release Builds

gdb support requires three things:

  1. The INTERNET permission.
  2. App Debugging enabled.
  3. An accessible gdbserver.

The INTERNET permission is enabled by default in Debug apps. If it is not already present in your application, you may add it either by editing Properties\AndroidManifest.xml or by editing the Project Properties.

App debugging can be enabled by either setting the ApplicationAttribute.Debugging custom attribute property to true, or by editing Properties\AndroidManifest.xml and setting the //application/@android:debuggable attribute to true:

<application android:label="Example.Name.Here" android:debuggable="true">

An accessible gdbserver may be provided by following the Debug Builds without Fast Deployment section.

One wrinkle: The _Gdb MSBuild target will kill any previously running app instances. This will not work on pre-Android v4.0 targets.

Troubleshooting

mono_pmip doesn't work

The mono_pmip function (useful for obtaining managed stack frames) is exported from libmonosgen-2.0.so, which the _Gdb target does not currently pull down. (This will be fixed in a future release.)

To enable calling functions located in libmonosgen-2.0.so, copy it from the target device into the gdb-symbols directory:

$ adb pull /data/data/Mono.Android.DebugRuntime/lib/libmonosgen-2.0.so Project/gdb-symbols

Then restart your debugging session.

Bus error: 10 when running the gdb command

When the gdb command errors out with "Bus error: 10", restart the Android device.

$ "/path/to/arm-linux-androideabi-gdb" -x "Project/gdb-symbols/gdb.env"
GNU gdb (GDB) 7.3.1-gg2
Copyright (C) 2011 Free Software Foundation, Inc.
...
Bus error: 10
$

No stack trace after attach

$ "/path/to/arm-linux-androideabi-gdb" -x "Project/gdb-symbols/gdb.env"
GNU gdb (GDB) 7.3.1-gg2
Copyright (C) 2011 Free Software Foundation, Inc.
...
(gdb) bt
No stack.

This is usually a sign that the contents of the gdb-symbols directory are not synchronized with your Android target. (Did you change your Android target?)

Please delete the gdb-symbols directory and try again.

Xamarin Workbook

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